The present invention relates to an electrical interconnection and means of making the interconnections, which are useful in electronic packaging applications such as in semiconductor integrated circuit chips and circuit boards and cards, cables and modules.
Recent developments in integrated circuits have clearly demonstrated the benefits which can be achieved by fabricating electrical devices into smaller and smaller packages. These small packages are densely packed, being multilevel with signal and power planes and other features on various levels and means of interconnecting selected levels to one another. The interconnections themselves provide sites for potential signal degradation. For example, interconnections between levels of conductor lines, and between conductor lines of a printed circuit board (PCB) or card and any electrical devices mounted thereon can be made between conductive areas called pads. Impedance matching, minimum number of discontinuities and redundancy must be present at these interconnections in order to permit rapid, low noise, low loss, low resistance signal transmission. Approaches used at present in devising interconnections for these electronic packages require several photolithographic masking and etching steps, and the attachment of surface mounted devices may require soldering and rework, often involving exposing the components to destructive temperature cycling.
Electrical interconnections comprised of interdigitated dendritic projections are a fertile field of scientific inquiry. The conical projections of the present invention are distinguished from dendritic projections by the method of making, by composition, by the controlled location and dimensions of conical projections. Schemes proposed to strengthen dendrites, such as coating with soft metal are described in IBM TDB Vol. 22, No. 7, p. 2706 by Babuka et al. and copending application Ser. No. 07/415,435 to Cuomo et al., filed Sep. 28, 1989 and commonly assigned with the present invention.
IBM Technical Disclosure Bulletin Vol. 22, No. 7, p. 2706, published December, 1979 by Babuka et al. describes a high density pad-to-pad connector on which dendrites are grown on a pad and coated with a liquid gallium alloy. When the dendritic pad is mated, the dendrites pierce the tarnished liquid metal film of a second pad and make the electrical contact.
IBM Technical Disclosure Bulletin, Vol. 24, No. 1A, June, 1981, p. 2, "Process For Producing Palladium Structures" by Armstrong et al. describes that the small cross-section of the base of the dendrite is at least partly to blame for breakage of dendrites. It also describes the need for "wipe" to make low resistance contact, but states that the roughness of the dendritic surfaces provides sufficient wipe.
IBM Technical Disclosure Bulletin, Vol. 23, No. 8, January, 1981, p. 1, "Dendrite Connector System With Reinforced Base" by Armstrong agrees with the above diagnosis, but differs in the proposed cure, proposing instead reflowing tin around the bases of the dendrites. Dendrites as pad-to-pad contact elements are also described in Research Disclosure, March, 1988, No. 287, p. 28748, "Method to Provide Multiple Dendritic Contact Points for High Density Flat on Flat Connector System", disclosed anonymously. Again, the dendrites, described are irregularly shaped and randomly located. However, the reduced connector length of the dendrites are described as providing noise reduction and improved signal speed, and the references suggests that having multiple contact points lowers contact resistance.
Other means in the art of making electrical interconnection between contact pads include spheres (U.S. Pat. No. 3,634,807 issued Jan. 11, 1972 to Grobe et al., U.S. Pat. No. 4,604,644 issued Aug. 5, 1986 to Beckham et al.) conductive rods (U.S. Pat. No. 4,644,130 issued Feb. 17, 1987 to Bachmann, U.S. Pat. No. 4,050,756 issued Sep. 27, 1977 to Moore, and U.S. Pat. No. 4,240,198, issued Dec. 23, 1980 to Alonso), hollow posts (U.S. Pat. No. 3,725,845) and third structures interposed between and parallel to the connector pads but separate from both (U.S. Pat. Nos. 3,881,799, issued May 6, 1975 to Elliott et al. and 3,634,807, issued Jan. 11, 1972 to Grobe et al.).
Flat-topped protrusions, permanently connecting pads between levels in a multilayer structure are described in the art (U.S. Pat. No. 4,751,563, issued Jun. 14, 1988 to Laibowitz et al.).
U.S. Pat. No. 3,634,807, issued Jan. 11, 1972 to Grobe et al. describes a removably attachable contact comprising a plurality of hollow metal spheres or wire balls mounted in a predetermined pattern on either side of a flexible insulating sheet. Alternatively, metal is deposited in openings at the intersection of thin strips of insulating material. In another embodiment, a conductive sheet is sandwiched between sets of contact elements. These embodiments are designed to be relatively inflexible in the X-Y direction and flexible in the Z direction. These structures are inadequate for high packing density structures, being of dimensions which are too large and too vulnerable to dirt contamination.
U.S. Pat. No. 3,725,845 issued Apr. 3, 1975 to Moulin describes a hermaphroditic connector comprising a plurality of hollow posts. This is a large scale connector for watertight use with cables in geophysical surveying, rather than for use with microminiature contact pads in packaging.
U.S. Pat. No. 3,881,799 issued May 6, 1975 to Elliott et al. describes a connector that comprises a plurality of domes projecting from both sides of a spring matrix, interposing a third element between the contacts to be connected, the third element is integral to neither.
U.S. Pat. No. 4,644,130 issued Feb. 17, 1987 to Bachmann describes a plurality of elastomeric connector rods which have been rendered conductive by being filled with conductive particles dispersed therein.
U.S. Pat. No. 4,751,563 issued Jun. 14, 1988 to Laibowitz et al. describes a method of making a cone shaped structure, having a carbonaceous surface contaminant, using an electron beam. A conductive layer is deposited on at least a portion of the cone and over the substrate area around the base of the cone. Then an insulating material is applied overall and any further processing is performed. Structures described in this patent are in the nature of through-holes, buried irreversibly within a unitary multilayer structure rather than being removably attached. Since electron beam radiation is used, the material from which the cone is comprised must be photoactive with respect to electron beams.
Unlike connectors described in the art, the connector of the present invention is reproducible, substantially smooth, of a single preselected height and non-brittle and applicable to interconnection of high density circuitry.
Independent of the electrical connection art, it is known that polyimide, along with certain other polymers, is removable by laser radiation (U.S. Pat. Nos. 4,328,410 issued May 4, 1982 to Slivinsky et al., 4,508,749 issued Apr. 2, 1985 to Brannon et al., and Research Disclosure, January, 1988, No. 285, p. 28,569, disclosed anonymously), and that irregular conical formations can inadvertently be obtained by laser ablation of a polyimide, ("Development and Origin of Conical Structures on XeCl Laser Ablated Polyimide", App. Phys. Letter 49(8), Aug. 25, 1986, pp. 453-455 by Dyer et al.). In the latter reference, cones were only formed in laser ablation of polyimide when impurities were present in the polyimide either unintentionally or were deliberately added as a "seed" of particulate impurities.
The Slivinsky et al. patent describes the removal of polyimide from a metallic substrate with lateral and depth precision and without causing damage to the metallic substrate. It further describes polyimide removal by a CO.sub.2 laser in the infrared region. Removal of polyimide by a CO.sub.2 laser in the infrared region proceeds by a mechanism different from ablation by excimer laser, such as in the present invention, which occurs in the ultraviolet region. Excimer lasers include XeCl, KrF, ArF and XeF lasers.
An improvement to the laser ablation rate of polyimide and other polymers by adding dye is described in Research Disclosure, January 1988, No. 285, p. 28,569, "Doped Polyimide Etching By Laser Ablation", disclosed anonymously. IBM TDB Vol. 30, No. 11, April 1988, p. 191, "Removal of Debris Left By Ablative Photodecomposition Of Polymers" by Braren et al. describes several ways to remove any debris that may be generated when polyimide or other polymer is removed by laser.
U.S. Pat. No. 4,508,749 to Brannon et al., commonly assigned with the present invention, describes the laser ablation of polyimide through a mask, exposing openings having sloped walls. The Brannon et al. patent is incorporated herein by reference for its description of excimer laser ablation of polyimide.
"Novel Method for Measuring Excimer Laser Ablation Thresholds of Polymers" by Dyer et al., published in App. Phys. Lett. 52 (22) May 30, 1988, pp. 1880-1882 describes control of morphological features, including cones, on laser ablated polyimide and other polymers which have, unlike the present invention, been "seeded". In neither of the Dyer et al. references is an electrical interconnection suggested or implied. "Structural Origin of Surface Morphological Modification Developed on Poly(ethylene terephthalate) (PET) by Excimer Laser Photoablation" by Novis et al., published in J. Appl. Phys. 64(1), Jul. 1, 1988, pp. 365-370 also describes seeding and ablation by XeCl lasers. However, the formation of cones in PI and PET during laser ablation is attributed to redeposition of debris, mostly carbon, in "The Effect of Debris Formation on the Morphology of Excimer Laser Ablated Polymers" by Taylor et al. in J. App. Phys. 64(5) Sep. 1, 1988, pp. 2815-2818.
In each of these journal publications, the formation of cones is regarded as undesirable, a phenomenon to be avoided.
Some processes used in fabricating electronic interconnections use chlorofluorocarbons (CFC's). Thus, it is one object of the invention to use laser mask technology rather than photolithography.
It is a further object of the invention to provide the capability to interconnect high density packages of electrically mounted devices and PCBs and/or cards to each other and to cables.
It is a further object of the invention to provide an electrical interconnection which permits reliable, rapid, dirt tolerant, low noise, low loss, low resistance signal transmission.
It is still a further object of the invention to provide a method of making the electrical interconnection described above in an efficient and controllable manner.
It is therefore an object of this invention to provide a conical electrical interconnection useful in the art of electronic packaging.
It is a further object of the invention to provide a conical, low resistance electrical interconnection which has a measure of flexibility and is nondestructively connectable and disconnectable.
Still another object of the invention is to provide a fabrication method to produce an electrical interconnection between two contact surfaces, at least one of which comprises essentially perpendicular conical projections of predetermined pattern and dimensions.
These and other objects, features and advantages of the present invention will become more apparent from the descriptions to follow.